Biology

The order Trogoniformes (trogons and quetzals) consists of a single family (Trogonidae), 6 genera, and 39 species. Most taxonomists recognize two subfamilies: Apalodermatinae (3 African species) and Trogoninae, which is split into tribe Harpactini (11 Asian species) and tribe Trogonini (25 New World species).^12,18,20

Trogonid distribution is roughly pantropical, with species approximately centered around the tropical forests of Malaysia and Indonesia and the equatorial forests of the Amazon and Congo river basins.^8,14 A variety of trogonids are found in Costa Rica, including the black-headed trogon (Trogon melanocephalus), Baird’s trogon (Trogon bairdii), the violaceous trogon (Trogon violaceus), the elegant trogon (Trogon elegans), the collared trogon (Trogon collaris), the orange-bellied trogon (Trogon aurantiiventris), the black-throated trogon (Trogon rufus), the slaty-tailed trogon (Trogon masena), the lattice-tailed trogon (Trogon clathratis), and the resplendent quetzal (Pharomacrus mocinno).^1,9

Trogonids use different sylvan habitats that range in elevation from sea level to more than 3,500 meters (m; 11,500 ft.). Although some species require primary forests, others flourish in secondary forest, forest fragment, logged forest, scrub, and agricultural land.¹⁴ The insectivorous red-headed trogon (Harpactes erythrocephalus) of Nepal inhabits middle- and lower-storey forests at elevations from 250 m up to 1830 m.¹⁰

Deforestation threatens trogonids worldwide. The Javan trogon (Apalharpactes reinwardtii) is on the International Union for the Conservation of Nature (IUCN) Red List as endangered. Twelve species are listed as near threatened. The resplendent quetzal (Pharomachrus mocinno), a species limited primarily to the remote and mist-draped cloud forests of middle America, is also listed by CITES (Appendix 1). The authors refer the readers to the IUCN website for more details, as the status of each species may change over time. Habitat loss, collection for zoos and aviaries, and the feather trade have raised concerns about this species’ status.⁸ Human activities that disrupt breeding, habitat, and food sources adversely affect trogonid populations.

Unique Anatomy

The foot anatomy of trogonids is described as heterodactylous, a term used for the toe arrangement in which the first and second toes are oriented posteriorly, with the hallux in the lateral position (heterodactylous toe). The anterior toes of most New World trogons are partially united, presumably an adaptation for nest excavation.¹⁴

Muscle distribution reflects aerial foraging. The heart is large, and the pectoral muscle complex accounts for approximately 20% of body weight, whereas the muscles of the relatively small feet and short legs represent only about 3%.^8,14,19 The muscles of the feet and legs are underdeveloped to the degree that trogonids are unable to turn around on a perch without the assistance of the wings.⁸ Walking and hopping are rare except in the elegant trogon.^8,14

Trogonid integument is bright and colorful, with soft, dense, and dry-textured feathers. Sexual dichromatism exists in all species except the Cuban trogon (Priotelus temnurus) and is most marked in the quetzals, in which, besides being more brightly colored, males also sport elongated and modified upper tail-coverts. These “tail feathers” are longest (48 to 96 centimeters [cm]) in the resplendent quetzal.¹⁴

Trogon is Greek for “to gnaw or eat” and refers to the structure and function of the beak. The cutting edges of the maxilla, the mandible, or both are variably serrated in most New World species and probably aid in securing live prey or large fruit. These serrations, along with the decurved tip of the bill (present in all species), are also useful in cutting food items into smaller pieces.^8,14 Most species have short, triangular tongues, with backward-pointing projections that probably aid in holding and swallowing prey.¹⁴ The Cuban trogon is an exception, having a relatively long tongue with a bifurcate tip that may be involved in nectar feeding.^7,8 All trogonids have short bills with an unusually wide base, which provides a large gape in relation to bill length and allows for ingestion of large food items.¹⁴ Resplendent quetzals, which regularly consume the large drupes (median diameter of 18 millimeters [mm]) of the laurel family (Lauraceae) that they have collected in flight, have several morphologic adaptations for a highly frugivorous diet.^1,19 In addition to a wide gape, this species has flexible mandibles and clavicles, which enable swallowing of fruits 3 to 4 mm wider than one would predict from gape measurements.¹⁹ The long esophagus (up to 12 cm) is thin walled, elastic, and ringed by circular muscles presumably important in regurgitation of large seeds; no crop is present. The proventriculus is expansible and lined with glandular tissue in a pattern of closely packed hexagons. The ventriculus is large (external diameter of 2.5 cm) and muscular.¹⁹ The paired ceca (each 4.5 cm long) are well developed and make up 15% total intestinal length, which suggests that some fermentative digestion occurs.¹⁴ The Cuban trogon, a primarily fruit-eating species, also lacks a crop and possesses a large ventriculus (1.8 cm diameter) and an even larger ceca (18% to 26% total intestinal length).^7,17 Examination of the gastrointestinal (GI) tracts of six other New World trogons also has revealed proportionally long ceca.^14,17

Special Physiology

In a review of avian metabolism, which compared over 350 species from more than 70 families, a low resting metabolic rate for the tropically adapted black-throated trogon (Trogon rufus) was reported.⁴ If typical of the family, limits on the climatic tolerances of all trogonids, both wild and captive, may exist. The distribution of New World species supports this theory. The relatively large 5 quetzal species and the eared trogon occupy mostly temperate high-altitude forests; the 8 mostly midsized species occupy intermediate altitude or temperature ranges; and the 9 smaller species occupy low-altitude tropical ranges.¹⁴

Housing and Feeding Requirements

Trogonids tend to be subcanopy or middle-strata hunters. They usually perch upright on horizontal branches between foliage and trunk and sit quietly for extended periods and move only their head through 180 degrees as they search for food and predators. The most common feeding behavior is termed “perch and pounce” or “sally-gleaning.” Birds plucks food items during graceful sallies without alighting beside fruit, plant, insect, or vertebrate prey.^1,8 Occasionally, some species such as the resplendent quetzal descend to the ground during pursuit of insects and lizards. Because of their weak feet and legs, trogonids have difficulty reaching for items from their perches, particularly those below them.^14,19 As such, trogonid enclosures should contain multiple horizontal branches, with some positioned over feeding stations, where the birds may perform sallies should they choose. Placement of plants and other structures within these areas should allow for unimpaired flight. Feed bowls should be shallow, with edges textured for perching so that the birds may alight and reach their food with minimal effort. Chronic pododermatitis has been reported in a pavonine (Pharomachrus pavoninus) and a golden-headed quetzal (Pharomachrus auriceps), and rotation of perches of different sizes and textures is recommended.

Trogonid diets vary from completely insectivorous to mostly frugivorous. In Africa, exclusive insectivory exists, presumably because of exclusion from other foods by other avian families early in trogonid evolution. Although they are predominantly insectivorous, many Asian species consume fruit and vegetation in moderate quantities. The greatest variation among trogonid diets is seen in the New World biogeographic zone, with its relatively large niche width and species divergence.⁸ Here, progression from exclusive or primary insectivory to primary frugivory directly correlates with increased body size and altitude and a decrease in insect life.¹² Progression during development also exists. The golden-headed quetzals are known to feed exclusively insects to their chicks for the first 3 days of the chicks’ lives. When this feeding strategy was employed in a group of these birds in captivity, digestive issues decreased, and chick survival increased.

Diets offered to captive trogonids consist of a mixture of vegetables (avocado, grape, apple, pear, melon, berries, banana, papaya, cactus, tomato, peas, corn, cooked potato, carrot) and vertebrate animal matter (pinky mice, hard-boiled egg, bird of prey diet), and insects (mealworms, waxworms, crickets, occasionally locusts). Pelleted diets have also been offered in some collections (soft bill diet, soaked dog food). Trogonids are susceptible to hemosiderosis and hemochromatosis, and limiting dietary iron should be considered. Trogonids must consume carotenoids to maintain their bright colorations,^6,13 and addition of a synthetic mixed carotenoid product to the diet is recommended.

The physiologic dependence on surface water is unknown, although drinking from a pool has been observed in wild elegant trogons.^8,15 Bathing has also been observed in wild elegant trogons,¹⁵ a wild Malabar trogon (Harpactes fasciatus),⁸ a captive golden-headed quetzal,⁵ and captive resplendent quetzals. On the basis of this information, provision of pool-type water sources is recommended.

Regular nail and beak trimming has been reported necessary for captive golden-headed quetzals. Providing vertically positioned soft or semi-decayed logs and food items sized such that the birds must cut them may decrease the need for these procedures and serve as enrichment by increasing foraging and digging behaviors.

Institutions in the United States that successfully breed some species of trogons have useful clinical databases on several adults and chicks. One of the more commonly identified problems with parent-reared chicks is parents feeding substrate materials to chicks within several days of hatching. In one of these institutions, chick losses involving white-tailed trogons (Trogon viridis) occurred most frequently within the first 14 days after hatching. Birds living past this age, in general, tended to do well and grew to a size and age that allowed safe transfer to other institutions.

Longevity

Trogonids may be long lived in captivity. Two resplendent quetzals housed at the Bronx Zoo lived for 17 and 21 years, respectively.¹⁴ One wild-caught golden-headed quetzal at the Houston Zoo lived for over 21 years, and a second housed at the Denver Zoo has reached 19 years of age. A breeding pair of white-tailed trogons (Trogon viridis) at the National Aquarium in Baltimore is over 16 years of age at the writing of this chapter. Historically, wild-caught animals have not fared as well in the United States. Historical acquisition or disposition information for United States institutions has revealed that 55% of wild-caught trogonids (80% quetzals) died within 1 year and 88% within 4 years.

Restraint and Anesthesia

Care must be taken when manually restraining trogonids because the skeleton is fragile, feathers are easily removed, and the skin tears easily.^8,13,18 Stress-induced death from physical restraint has occurred in healthy trogonids; therefore, the birds’ reactions should be closely observed.¹³ Use of inhalant anesthesia with isoflurane has been reported in captive trogonids; this agent should be used for invasive procedures or for birds that struggle excessively.¹⁷ Captive-reared and hand-reared individuals appear to handle restraint better overall compared with their wild counterparts.

Diagnostics

Diagnostic testing on trogonids is similar as in other avian species. Hematologic and plasma biochemical reference ranges for selected species are listed in Table 28-1.

TABLE 28-1

Reference Ranges for Hematologic and Plasma Biochemical Parameters of Selected Captive Trogonid Species

Parameter	Golden-Headed Quetzal (Pharomachrus auriceps) (N)	Crested Quetzal (P. antisanus) (N = 1)	White-Tailed Trogon (Trogon viridis) (N)	Blue-Tailed Trogon (Harpactes reinwardti) (N = 1)
Leukocytes ×103/µL	5.484 + 5.384 (11)	8.800 + 0	J 5.693 + 1.879 (12) A 6.27 + 2.055 (7)	12.485 + 0
Heterophils ×103/µL	2.059 + 1.626 (11)	6.160 + 0	J 1.336 + 616 (13) A 2.311 + 689 (7)	7.491 + 0
Lymphocytes ×103/µL	3.236 + 3.032 (11)	2.288 + 0	J 2.578 + 1.432 (13) A 1.902 + 0.974 (7)	2.622 + 0
Monocytes ×103/µL	0.534 + 0.607 (11)	0.352 + 0	J 0.253 + 0.163 (11) A 0.535 + 0.173 (6)	0.749 + 0
Eosinophils ×103/µL	0.317 + 0.426 (11)	0.000 + 0	J 0.396 + 0.176 (12) A 1.284 + 0.395 (5)	0.874 + 0
Basophils ×103/µL	0.067 + 0.110 (11)	0.000 + 0	J 1.018 + 0.334 (11) A 0.916 + 0.429 (7)	0.749 + 0
Erythrocytes ×106/µL	3.53 + 0.45 (6)	1.41 + 0	2.49 + 0.360	—
PCV (%)	52.3 + 4.4 (7)	40 + 0	J 52.6 + 2.55 (13) A 55.4 + 4.5 (6)	50 + 0
Hemoglobin (g/dL)	17.1 + 1.9 (6)	12.9 + 0	J 19.85 + 0.7 A 18.2 + 5	—
MCV (fL)	152.5 + 12.1 (4)	284.7 + 0	193 + 25.5	—
MCH (mg/dL)	48.7 + 4.9 (6)	91.8 + 0	—	—
MCHC (µg)	31.5 + 3.7 (4)	32 + 0	J 38 + 1.5 (11) A 32.5 + 4.5 (2)	—
Total protein (g/dL)	2.9 + 0.8 (9)	5.2 + 0	J 3.6 + 0.38 (13) A 3.73 + 0.31 (6)	3.9 + 0
Albumin (g/dL)	1.0 + 0.2 (3)	—	J 2 + 0.23 (6) A 0.96 + 0.15 (3)	0.8 + 0
Globulin (g/dL)	1.8 + 0.1 (2)	—	A 1.6 + 0.2 (3)	—
Calcium (mg/dL)	9.1 + 0.5 (6)	15.1 + 0	J 9 + 0.6 (16) A 8.4 + 0.5 (3)	11.8 + 0
Phosphorus (mg/dL)	3.5 + 1.6 (3)	—	J 4.5 + 1.2 (16) A 3.8 + 0.1 (3)	3.7+ 0
Sodium (mEq/L)	158 + 4 (6)	—	J 155 + 3.9 (16) A 157 + 1 (3)	—
Potassium (mEq/L)	2.4 + 1.1 (4)	—	J 3 + 0.86 (14) A 2.4 + 0 (3)	—
Chloride (mEq/L)	121 + 2.3 (2)	—	A 123 + 2.5 (2)	—
Creatinine (mg/dL)	0.1 + 0 (1)	—	—	—
Urea nitrogen (mg/dL)	2 + 0 (1)	—	—	—
Cholesterol (mg/dL)	276 + 17 (3)	—	—	370 + 0
Triglycerides	293 + 111 (3)	—	—	—
Glucose (mg/dL)	245 + 84 (6)	288 + 0	339 + 0	332 + 0
			J 292 + 24 (17) A 354 + 75 (4)
Total carbon dioxide	18 + 0 (1)	—	—	—
Manganese (mEq/L)	—	—	—	2.6 + 0
ALP (Units/L)	231 + 179 (4)	—	A 69 + 40 (3)	83 + 0
AST (Units/L)	93 + 20 (7)	—	203 + 69	263 + 0
			J 191 + 48 (17) A 210 + 67 (4)
ALT	26 + 10 (4)	—	A 47 + 28 (3)	81 + 0
LDH (Units /L)	215 + 132 (3)	—	A 99 + 30 (3)	265 + 0
CPK (Units /L)	450 + 472 (3)	—	J 295 + 95 (13) A 245 + 201 (4)	703 + 0
GGT (Units /L)	7 + 0 (1)	—	A 8.3 + 3.2 (3)	6 + 0
Uric acid (mg/dL)	11.0 + 4.8 (6)	—	J 6 + 1.9 (11) A 8.5 + 3.8 (4)	8.6 + 0
Total bilirubin (mg/dL)	0.4 + 0.2 (4)	—	—	—
Amylase (Units /L)	324 + 26 (3)	—	—	—
Reference (Unpublished data)	Houston Zoo, Denver Zoo, Philadelphia Zoo	Philadelphia Zoo	National Aquarium in Baltimore	San Diego Zoo

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